Doping of semiconductor substrates with conductivity-determining type impurities, such as n-type and p-type ions, is used in a variety of applications that require modification of the electrical characteristics of the semiconductor substrates. Well-known methods for performing such doping of semiconductor substrates include photolithography and screen printing. Photolithography requires the use of a mask that is formed and patterned on the semiconductor substrate. Ion implantation then is performed to implant conductivity-determining type ions into the semiconductor substrate in a manner corresponding to the mask. Similarly, screen printing utilizes a patterned screen that is placed on the semiconductor substrate. A screen printing paste containing the conductivity-determining type ions is applied to the semiconductor substrate over the screen so that the paste is deposited on the semiconductor substrate in a pattern that corresponds inversely to the screen pattern. After both methods, a high-temperature anneal is performed to cause the impurity dopants to diffuse into the semiconductor substrate.
In some applications such as, for example, solar cells, it is desirable to dope the semiconductor substrate in a pattern having very fine lines or features. The most common type of solar cell is configured as a large-area p-n junction made from silicon. In one type of such solar cell 10, illustrated in FIG. 1, a silicon wafer 12 having a light-receiving front side 14 and a back side 16 is provided with a basic doping, wherein the basic doping can be of the n-type or of the p-type. The silicon wafer is further doped at one side (in FIG. 1, front side 14) with a dopant of opposite charge of the basic doping, thus forming a p-n junction 18 within the silicon wafer. Photons from light are absorbed by the light-receiving side 14 of the silicon to the p-n junction where charge carriers, i.e., electrons and holes, are separated and conducted to a conductive contact, thus generating electricity. The solar cell is usually provided with metallic contacts 20, 22 on the light-receiving front side as well as on the back side, respectively, to carry away the electric current produced by the solar cell. The metal contacts on the light-receiving front side pose a problem in regard to the degree of efficiency of the solar cell because the metal covering of the front side surface causes shading of the effective area of the solar cell. Although it may be desirable to reduce the metal contacts as much as possible so as to reduce the shading, a metal covering of approximately 5% remains unavoidable since the metallization has to occur in a manner that keeps the electrical losses small. In addition, contact resistance within the silicon adjacent to the electrical contact increases significantly as the size of the metal contact decreases. However, a reduction of the contact resistance is possible by doping the silicon in narrow areas 24 directly adjacent to the metal contacts on the light-receiving front side 14.
FIG. 2 illustrates another common type of solar cell 30. Solar cell 30 also has a silicon wafer 12 having a light-receiving front side 14 and a back side 16 and is provided with a basic doping, wherein the basic doping can be of the n-type or of the p-type. The light-receiving front side 14 has a rough or textured surface that serves as a light trap, preventing absorbed light from being reflected back out of the solar cell. The metal contacts 32 of the solar cell are formed on the back side 16 of the wafer. The silicon wafer is doped at the backside relative to the metal contacts, thus forming p-n junctions 18 within the silicon wafer. Solar cell 30 has an advantage over solar cell 10 in that all of the metal contacts of the cell are on the back side 16. In this regard, there is no shading of the effective area of the solar cell. However, for all contacts to be formed on the back side 16, the doped regions adjacent to the contacts have to be quite narrow.
As noted above, both solar cell 10 and solar cell 30 benefit from the use of very fine, narrow doped regions formed within a semiconductor substrate. However, the present-day methods of doping described above, that is, photolithography and screen printing, present significant drawbacks. For example, it is prohibitively difficult, if not impossible, to obtain very fine and/or narrow doped regions in a semiconductor substrate using screen printing. In addition, while doping of substrates in fine-lined patterns is possible with photolithography, photolithography is an expensive and time consuming process. In addition, both photolithography and screen printing involve contact with the semiconductor substrate. However, in applications such as solar cells, the semiconductor substrates are becoming very thin. Contact with thin substrates often results in breaking of the substrates. Further, screen printing cannot be used to dope rough or textured surfaces, which are commonly used in solar cell design to trap light within the semiconductor substrate. Moreover, because photolithography and screen printings use custom designed masks and screens, respectively, to dope the semiconductor substrate in a pattern, reconfiguration of the doping pattern is expensive because new masks or screens have to be developed.
Accordingly, it is desirable to provide boron-comprising inks for forming boron-doped regions in semiconductor substrates using non-contact printing processes. It also is desirable to provide methods for fabricating boron-comprising inks for forming such boron-doped regions using non-contact printing. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.